1. Field of the Invention
The present invention relates to a photonic semiconductor device and a method for fabricating the device. More particularly, the invention relates to a semiconductor laser device for use with wavelength division multiplexing techniques for implementing optical communications, as well as to a method for fabricating the device.
2. Description of the Related Art
In recent years, wavelength division multiplexing (WDM) techniques have been drawing attention for their ability to boost capacities of data transmission over optical fibers. The use of WDM can appreciably multiply the capacity of an installed optical fiber to transmit data by a factor of dozens. Distributed feedback laser diodes (DFB-LD) used as light sources of WDM systems are required to keep their oscillation wavelengths at uniform intervals of, say, 0.4 or 0.8 nanometers (nm) within a wide wavelength range of, illustratively, between 10 and 50 nm. The requirement has been met conventionally by use of semiconductor laser units each acting as a light source corresponding to a specific oscillation wavelength. This practice has gotten costlier as a larger number of light sources is installed.
The growing cost of configured light sources is making it progressively unfeasible to have more and more wavelengths multiplexed. One solution to this problem is to utilize a wavelength tunable LD capable of producing many wavelengths by varying the electric current of a single chip. Another solution is the use of a plurality of LDs arrayed in a single integrated chip for dealing with numerous wavelengths. These light sources are generically referred to as multi-wavelength laser sources herein.
It is preferred that the multi-wavelength laser source be used not only as a light source of an actual system but also as a backup light source of WDM transmission equipment wherein numerous light sources are supported by a single chip. Such a setup if implemented is advantageous in terms of cost as it constitutes an inexpensive but highly reliable transmission system.
In that respect, inexpensive multi-wavelength laser sources for routing information to different locations on a network by varying laser output wavelengths (i.e., wavelength routing) are expected to play a major role in constructing a fully optical network in the future. Studies are underway, for multi-wavelength laser sources, on such devices as wavelength tunable LDs for addressing many wavelengths by varying the electric current value on a single chip, as well as a plurality of LDs arrayed into a single chip for dealing with numerous wavelengths, each device being integrated with light modulators.
Illustratively, authors of xe2x80x9cCompact High-Power Wavelength Selectable Laser for WDM Applicationsxe2x80x9d (Optical Fiber Communication Conference Technical Digest, Tull, Mar. 5-10, 2000, Baltimore, Md., U.S.A.) disclose a chip that integrates an eight-channel laser array with outputs of eight wavelengths arranged at intervals of 3.18 nm, a combiner for combining the multiple outputs into one output, and a semiconductor optical amplifier. The combiner for selecting one of the eight wavelengths in the chip is a 8xc3x971 MMI (multi-mode interference) wave-guide combiner.
FIG. 12 is a perspective view of a conventional semiconductor laser device. In FIG. 12, reference numeral 200 stands for a semiconductor laser device; 202 for a laser unit; 204 for a combiner unit; 206 for an output unit; 208 for a laser array; 210 for electrodes of the laser array 208; 212 for a 4xc3x971 MMI combiner; 214 for an electro-absorption modulator (EAM); 216 for electrodes of the EAM 214; and 218 for an InP burial layer.
A typical method for fabricating the conventional semiconductor laser device is outlined below. FIG. 13 is a perspective view of the conventional laser device in one process of its fabrication. FIG. 14 is a perspective view for schematically explaining how the conventional semiconductor laser device can develop a defect attributable to the method for fabricating that device.
In FIG. 13, an n-InP clad layer 222 (n-conductivity type is referred to as xe2x80x9cn-xe2x80x9d hereunder), a laser active layer 224, and a p-InP clad layer 226 (p-conductivity type is referred to as xe2x80x9cp-xe2x80x9d hereunder) are formed on an InP substrate 220. From the substrate 220, portions except for the laser unit 202 are then removed. Over the substrate regions cleared of their layer elements, an n-InP clad layer 228, an optical wave-guide layer 230 and a p-clad layer 232 are formed. During the process, a diffraction grating layer (not shown) is formed within the n-InP clad layer 222 or p-InP clad layer 226 of the laser unit 202.
An insulating film is then formed over the layered structure. A mask pattern 234 is prepared through which to form the laser unit 202 as a band-shaped laser array 1 to 2 xcexcm wide, the MMI combiner 212 as a rectangle 5 to 50 xcexcm wide and 20 to 500 xcexcm long in the direction of a resonator, and the light modulator as a band shape 1 to 2 xcexcm wide. Etching is carried out using the mask pattern 234 as a mask until the laser active layer 224 of the laser unit 202 or the optical wave-guide layer 230 is etched through or until the substrate 220 is exposed, whereby a ridge structure is formed. FIG. 13 shows the outcome of the processing. Later, a buried growth process is performed on the InP burial layer 218 using the mask pattern 234 as a mask for selective growth.
The contact electrodes 210 are then formed on the laser unit 202, and a contact electrode 216 is furnished to the EAM 214 of the output unit 206. A back surface of the substrate 220 is polished to a thickness of 100 xcexcm to form back surface electrodes. This completes the semiconductor laser device 200 illustrated in FIG. 12.
One disadvantage of the above process of fabricating the semiconductor laser device 200 is this: because of an extensive top surface of the MMI combiner 212, the buried growth process by use of the selective growth mask can leave InP polycrystals 238 grown over the insulating film on the wave-guide of the MMI combiner 212 as shown in FIG. 14. The polycrystals thus grown can result in broken resist films or related irregularities in subsequent processes.
FIG. 15 is a perspective view of another conventional semiconductor laser device. In FIG. 15, reference numeral 240 stands for a semiconductor laser device and 242 for branching wave-guides. The semiconductor laser device 240 utilizes the branching wave-guides 242 in place of an MMI combiner 212.
FIG. 16 is a perspective view of the conventional laser device of FIG. 15 in one process of its fabrication. FIG. 17 is a perspective view for schematically explaining how the conventional semiconductor laser device can develop a defect attributable to the method for fabricating that device.
The method for fabricating the semiconductor laser device 240 is the same as the method for producing the semiconductor laser device 200 in forming the layered structure of the laser unit 202, combiner 204 and output unit 206.
An insulating film is then formed over the layered structure. A mask pattern 244 is prepared through which to form the laser unit 202 as a band-shaped laser array 1 to 2 xcexcm wide, the branching wave-guides 242 to a width of 1 to 2 xcexcm each connected to the laser unit array, and the light modulator as a band shape 1 to 2 xcexcm wide. Etching is carried out using the mask pattern 244 as a mask until the active layer of the laser unit 202 or wave-guide layers of other regions are etched through, whereby a ridge structure is formed. FIG. 16 shows the outcome of the processing.
Later, a buried growth process is performed on the InP burial layer 218 using the mask pattern 244 as a mask for selective growth.
Contact electrodes 210 are then formed on the laser unit 202, and a contact electrode 216 is furnished to the EAM 214 of the output unit 206. A back surface of the substrate 220 is polished to a thickness of 100 xcexcm to form back surface electrodes. This completes the semiconductor laser device 240 shown in FIG. 15.
A disadvantage of the above process of fabricating the semiconductor laser device 240 is this: when the buried growth process is carried out on the InP burial layer 218 to form the branching wave-guides 242 in the laser unit 202 and output unit 206, bases of the branches can develop projections 246 through abnormal growth, as shown in FIG. 17. The projections 246 can result in broken resist films or related irregularities in subsequent processes.
The buried growth process of the combiner portion, when performed conventionally, can entail formation of polycrystals 238 or projections 246 through abnormal growth as described above. The defect leads to faulty processes that can end up lowering the yields of the photonic semiconductor device or degrading its reliability.
Publications related to this invention include Japanese Published Unexamined Patent Application No. Hei 11-211924. The publication discloses a silicon substrate arrangement carrying: a plurality of cores for propagating light emitted by a plurality of single longitudinal mode semiconductor chips incorporating electro-absorption semiconductor light modulators; a multi-mode interference wave-guide combiner; and at least one output crystal optical wave-guide. The disclosure, however, makes no reference to a buried structure of the multi-mode interference optical combiner.
The present invention has been made in view of the above circumstances, and a first object of the invention is therefore to overcome the above and other deficiencies of the prior art and to provide a highly reliable photonic semiconductor device that is fabricated with high yield rates.
According to one aspect of the invention, there is provided a photonic semiconductor device comprising: a laser unit made of a plurality of single wavelength semiconductor lasers each of which has a different wavelength and comprises a pair of current block structures sandwiching from both sides an optical wave-guide ridge including an active layer; an output unit having a first wave-guide layer sandwiched from above and below by a first upper clad layer and a first lower clad layer, and outputting a laser emission coming from the laser unit; a combiner unit having a second wave-guide layer sandwiched from above and below by a second upper clad layer and a second lower clad layer, and having one end connected to the laser unit and the opposite end connected to the output unit; a semiconductor substrate for carrying the laser unit, the output unit and the combiner unit; and a burial semiconductor layer of a material composition identical to that of the current block structures of the laser unit, which is disposed on the semiconductor substrate, and which covers and buries the combiner unit.
Accordingly, the inventive structure constitutes a highly reliable photonic semiconductor device fabricated at low costs with high yield rates.
Another object of the invention is to provide a method for fabricating a highly reliable photonic semiconductor device with high yield rates.
According to another aspect of the invention, there is provided a photonic semiconductor device fabricating method comprising the steps of: performing a first process wherein a semiconductor layer for a semiconductor laser unit is first deposited on a semiconductor substrate; a semiconductor laser layer which is a part of the semiconductor layer is then left on the substrate by removing the rest of the semiconductor layer; a combiner unit layer having a second wave-guide layer sandwiched from above and below by a second upper clad layer and a second lower clad layer is formed connecting to the semiconductor laser layer; and an output unit layer having a first wave-guide layer sandwiched from above and below by a first upper clad layer and a first lower clad layer is formed connecting to the combiner unit layer; performing a second process wherein a dielectric film is formed over surfaces of the semiconductor laser layer, the combiner unit layer, and the output unit layer; photolithographic and etching techniques are used to form a plurality of stripe-shaped mask patterns on the semiconductor laser layer, a prescribed mask pattern on the combiner unit layer, and a stripe-shaped mask pattern on the output unit layer; and etching is carried out using the mask patterns as masks so as to form a plurality of optical wave-guide ridges of the semiconductor laser unit, a combiner unit, and an output unit ridge; and performing a third process wherein the dielectric film is removed from the combiner unit so as to have a mask pattern formed by the remaining dielectric film; and the mask pattern is used as a selective growth mask through which the combiner unit is covered with and buried by a semiconductor layer constituting current block structures of the semiconductor laser unit.
Accordingly, in forming the combiner unit by buried growth, this fabricating method prevents polycrystals or like abnormal projections from growing so that subsequent processes are carried out with no trouble. The inventive method thus makes it possible to fabricate a highly reliable photonic semiconductor device in simplified steps with high yield rates.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.